Brushless direct current (BLDC) motors are becoming more prevalent in industries that typically did not use BLDC motors. For example, the need for increased efficiency in the heating and air conditioning market has led to the use of BLDC motors for powering the blower. BLDC motors include a rotor having a plurality of magnetic poles (e.g., a plurality of poles produced with permanent magnets) of alternating polarity disposed on a surface of a rotor core, and a stator that receives electrical power and produces a magnetic field in response thereto. The magnetic field of the stator interacts with a magnetic field of the rotor to cause movement of the rotor.
BLDC motors require a means for determining the position of the rotor in order to commutate the motor. One method of commutating the motor is referred to as “sensorless” motor commutation. Sensorless motor commutation is often performed by sensing the back electromotive force (BEMF) produced by the motor. Typically, the BEMF signal produced in the stator windings is not large enough for sensorless motor commutation until the speed of the rotor reaches about ten percent of the rated motor speed. As a result, a means of starting the motor without using the BEMF signal may be necessary.
For a three-phase motor, one method of starting the motor is to align the rotor by providing current to one phase of the motor and wait until the rotor has stopped oscillating, then step through the other phases of the motor (with each subsequent phase getting shorter, thus ramping the speed up without any position feedback) until the rotor reaches 10% of rated speed. This method traditionally has two drawbacks. First, the time required during the align phase can be long where the inertia of the attached load is large and the friction is low (e.g., if the load is a large blower). Second, information about the load (e.g., inertia and torque) is typically required in order to step the motor.
The purpose of aligning the rotor as described earlier is to start the motor from a known rotor position. One way to avoid this aligning process is by knowing the rotor position by some other method. The second drawback described earlier can be overcome by not stepping blindly (without rotor position information) but by knowing the rotor position at almost zero speed.
In one embodiment, the invention provides a method of controlling an electrical machine having a stator and a rotor. The stator includes a core and a plurality of windings disposed on the core in a three-phase arrangement. The three-phase arrangement includes a first phase, a second phase, and a third phase having a first terminal, a second terminal, and a third terminal, respectively. The rotor is disposed adjacent to the stator to interact with the stator. The method includes the steps of applying a pulsed voltage differential to the first and second terminals resulting in movement of the rotor; monitoring the back electromotive force (BEMF) of the third phase to sense rotor movement; after the applying and monitoring steps, monitoring the BEMF of each of the first, second, and third phases to determine the direction of rotation of the rotor; determining whether the rotor is rotating in a desired direction, and electrically commutating the motor when the rotor is rotating in the desired direction and zero or more other conditions exist.
In another embodiment, the invention provides a method of controlling an electrical machine having a stator and a rotor. The stator includes a core and a plurality of windings disposed on the core in a multiphase arrangement. The rotor is disposed adjacent to the stator to interact with the stator. The method includes, prior to purposely causing movement of the rotor, sensing a BEMF of at least one of the phases, determining whether the rotor is moving based on the sensed BEMF, defining a state of the motor (e.g., a no moving state, a slow moving state, and a fast moving state), and stopping movement of the rotor if the motor falls under a slow moving state. The method can further include starting movement of the rotor as discussed above.
In yet another embodiment, the invention provides a method for controlling an electrical machine having a stator and a rotor. The stator includes a core having a plurality of phase windings disposed on a core. The rotor is disposed adjacent to the stator and includes a plurality of magnetic poles. The method includes initiating an aligning process of the stator and the rotor by generating a moving force to cause rotation of the rotor with respect to the stator and generating a braking force to at least slow rotation of the rotor with respect to the stator. The generating of a moving force to cause rotation of the rotor can include exciting at least one of the phase windings to generate an attracting magnetic force between the excited at least one phase winding and at least one of the magnetic poles, and the generating of a braking force to at least slow rotation of the rotor can include exciting at least one of the phase windings to generate a force opposite to the rotational direction of the rotor with respect to the stator. The method may also include alternating between generating the moving force and generating the braking force. The method may further include defining a specific amount of time to align the stator and the rotor, where the specific amount of time may include a plurality of cycles such as an exciting cycle, a braking cycle, and a coast cycle.
In a further embodiment, the invention provides a method for controlling an electrical machine with a stator and a rotor. The stator includes a core with a plurality of phase windings disposed on the core. The rotor is disposed adjacent to the stator and includes a plurality of magnetic poles. The method includes generating a moving force to cause rotation of the rotor with respect to the stator, and generating a braking force to at least slow rotation of the rotor with respect to the stator. The method also includes alternating the generating a moving force and the generating a braking force for a period of time, and stopping rotation of the rotor at one of one or more known rotor positions.
In another embodiment, the invention provides a method of controlling an electrical machine with a stator having a core and a plurality of windings disposed on the core in a multiple phase arrangement, and a rotor disposed adjacent to the stator to interact with the stator. The method includes applying a first pulsed voltage to a first terminal of a first phase of the multiphase arrangement, monitoring back electromotive force (BEMF) of at least one phase of the multiphase arrangement, and determining a peak value of BEMF. The method also includes obtaining a first monitored value of BEMF, comparing the peak value of BEMF against the first monitored value of BEMF, and determining whether the rotor is rotating based of the comparison.